1. Field of Invention
This invention relates to ink jet printers.
2. Description of Related Art
A thermal ink jet print head selectively ejects droplets of ink from a plurality of drop emitters to create a desired image on an image receiving member, such as a sheet of paper. The print head typically comprises an array of the drop emitters that convey ink to the image receiving member. In a carriage-type ink jet print head, the print head moves back and forth relative to the image receiving member to print the image in swaths. Each swath can be many pixels tall.
An ink jet print head typically comprises a plurality of ink passageways, such as capillary channels. Each channel has a nozzle and is connected to an ink supply manifold. Ink from the manifold is retained within each channel until, in response to an appropriate signal applied to a resistive heating element in each channel, the ink and a portion of the channel adjacent to the heating element is rapidly heated and vaporized. Rapid vaporization of some of the ink in the channel creates a bubble that causes a quantity of ink, i.e., an ink droplet, to be ejected from the emitter to the image receiving member.
The productivity of ink jet color printing has always been an important factor in user satisfaction. The low color productivity is generally a result of the fact that the ink jet print head carries out multi-pass printing to achieve a high level of print quality. Current state of the art ink jet printers offer users a choice of fast color printing or good quality color printing, but both requirements can not be simultaneously achieved. This choice of speed or quality comes from selecting either the DRAFT mode or normal and/or high quality mode. In DRAFT mode, printing is done is a single pass and with less than 100% inking. For example, DRAFT mode printing often employs a 50% checker board mask to reduce ink coverage by xc2xd. As a result, the color images are created quickly, but they do not have saturated, deep color and they also appear very grainy. In the normal or high quality modes, printing is done in multiple passes and with full ink saturation. While the output is delivered more slowly, the quality is very high and often surpasses that achieved by much more expensive printing methods and apparatus.
This invention allows a scanning carriage type ink jet printer to produce high quality color output while using a bi-directional, single pass printing method. The term xe2x80x9cbi-directionalxe2x80x9d refers to the printing technique where printing takes place while the carriage is scanning in both directions. The term xe2x80x9csingle passxe2x80x9d means that all of a particular color is deposited on the media with a single pass of the color print head over an area. Such a printing method allows up to 2 times faster color printing from the same number of color jets. Since jet count increase adds to printer cost, this method is a low cost way to achieve high color print speed.
While the current single-pass printing methods produce fast output, the image formed by this technique suffers from significantly reduced print quality. The reduced print quality is a consequence of unsaturated printing. One possible way to increase single pass print quality is to completely saturate the printed color area in one pass. When single pass printing with full saturation is attempted, inter-color bleeding (ICB) between the black and one or more of the color inks on the recording medium makes the output quality of color prints unacceptable. Inter-color bleeding results when one ink, usually the slow drying ink, wicks along paper fibers that have been wetted by another ink. In many more serious situations, the slow dry ink, due to the fact that it sits on the paper""s surface, moves significant distances into adjacent color areas. This bleeding image defect is most objectionable when yellow is printed next to black, but is also visible with other colors.
Factors that affect inter-color bleeding include: (1) ink properties such as surface tension of the inks and interactions between the inks; (2) the dry time of the inks, where the shorter the dry time the lower the inter-color bleeding becomes; (3) the time between the two bleeding inks being printed, where the longer the delay the lower the inter-color bleeding becomes; and (4) the amount of ink available to feed the bleeding. In practice, the worst situation for inter-color bleeding combines a slow dry ink, a large drop volume, and a solid area image. Most ink jet printers make use of slow dry or high hold out formulations for the black ink and fast drying or fast absorbing formulations for the color inks. These black and color ink choices are made because it is not possible to get the required level of black area darkness and edge sharpness with fast dry inks on a range of plain papers. Dark, sharp black printing is required for ink jet to achieve laser quality black printing.
The most common prior art ink jet print head configuration for color printing is for the three or more color jet arrays to be arranged in parallel rows. The color sections are typically cyan, magenta and yellow. With this configuration, strokes of different colors are printed in the same print swath. This color print head configuration will be called xe2x80x9csame swath color print headsxe2x80x9d to differentiate this prior art color print head configuration from the configuration employed in the present invention.
A second black print head has a jet array that is about 3 or more times the length of the color array. The print heads are mounted on the scanning carriage of the printer to place the color arrays vertically (perpendicular to the scan direction) so that they terminate in the same place as one end of the black array. The common black and color array vertical termination point is at the end of the array where printing first takes place. In this configuration, the full stroke of color and black arrays would appear on the media as parallel strokes with the black extending 2 to 3 times further than the color strokes. This color and black print head configuration is typical of products such as the HP 800 series and 900 series ink jet printers.
An alternative configuration is for the color arrays within a tri-color print head to be arranged in a line so that they form a parallel array next to the black array. In the case of parallel arrays, the length of the combined color arrays is typically shorter than the black array.
In the present invention, the color arrays are arranged sequentially rather than in parallel so that each of their strokes would appear in a subsequent print swath. In addition, the black print head is offset from one color section of the tri-color print head, so that one-third of the black print head is behind the tricolor print head in the direction of travel of the recording medium.
Prior art ink jet printers carry out high quality color printing by printing all of the color first in two or more passes and then printing all of the black in one pass with the last section of the black print head. This printing method achieves the longest delay between color and black printing. In order for this printing method to produce acceptable print quality, and even with the long delay that is achieved with this configuration, special additives must be included in the color ink formulation in order for bleeding to be reduced to an acceptable level. These additives are typically precipitating agents that are added to the color inks. The precipitating agents precipitate the black colorant as it moves toward the color sections.
The use of tri-color print heads that have all colors printing in one swath has the further problem that the order of color printing is reversed for right to left with respect to left to right printing. The color print order reversal with scan direction change leads to color tone changes when secondary colors are printed. Therefore, for same swath tri-color print heads, a second problem of color banding also produces single pass color print quality defects.
Since the black ink is a slow drying ink, the color inks are printed first. In one configuration, the yellow ink is printed three swaths ahead of the black ink. The magenta ink is printed two swaths ahead of the black ink. The cyan ink is printed one swath ahead of the black ink. The yellow section is located the furthest from the black to allow more time for the yellow ink to dry because the black to yellow inter-color bleeding is most visible.
Unfortunately, this approach still gives a very poor inter-color bleeding print quality. This technique also results in a non-uniform usage of the black print head because the offset one-third of the black print head that is not aligned with the color print head will be used much more than the remaining two-thirds of the black print head. Over long term use, this will result in reduced print head lifetime and eventually banding effects in the printed image.
This invention utilizes a unique color and black drop ejector array configuration in conjunction with a specific color printing order and black ink lay down method.
Accordingly, this invention provides ink jet printer systems and methods that reduce inter-color bleeding.
This invention separately provides ink jet printing systems and methods that reduce inter-color bleeding without substantially affecting productivity.
In various exemplary embodiments, a printing systems and methods reduce an amount of inter-color bleeding between the black ink and one or more of the color inks by distributing the printing of a total amount of the black ink required in a single swath over multiple passes while, each of the color inks required for the same swath may be printed in a single pass. For example, in various exemplary embodiments, the printing systems and methods may print a single swath by printing portions of the slow dry black ink in three passes while printing each of the fast dry color inks in single passes.
While many of the examples for using this invention are made with reference to thermal inkjet drop ejection methods here, this invention is broadly applicable to any method for printing with liquid ink drops. Examples of other methods include continuous ink jet, piezo-electric inkjet and MEM""s based ink jet. All of these liquid printing methods are within the scope of this invention.